In a conventional projection exposure apparatus utilized in a fabricating process of semiconductor devices, as shown in FIG. 13, a photo mask 52 that is disposed at a focusing position on an optical axis is radiated with a light from a light source 51. Then, a pattern of the photo mask pattern 52 thus radiated is projected onto and exposed to a photoresist on a wafer 54 (or a variety of thin films formed on the wafer 54) by means of a projection lens 53.
As shown in FIG. 14(a), the photo mask 52 is formed such that a pattern 522 of a film made of a light-shielding material such as a chromium film is formed on the surface of a glass substrate 521 made of a material such as a quartz glass.
When such a photo mask 52 is utilized, laser beams passed through transmission portions 523 where no light-shielding film 522 is provided are spread wider than the mask pattern on the wafer 54 because of the restriction by the resolution limit in the projection lens 53. Since the passed laser beams are the same in phase, a mutual action acts to increase their intensities, thereby producing an intensity-distribution on the wafer 54 such that, as shown in FIG. 14(c), two adjacent portions of the pattern cannot be separated. In the case of the reduction projection, the pattern of the photo mask 52 is reduced and then projected onto the wafer. The widths of the light shielding portion 522 and the transmission portion 523 are generally wider than 0.3 to 0.5 .mu.m, when converted to the dimensions on the wafer.
Recently, a phase shift method which can improve a resolution only by changing the photo mask without changing the projection exposure apparatus receives a remarkable attention. The phase shift method is described, for example, in 24th VLSI Forum, the Front of Lithography, "Present Situation of Phase Shift Lithography" reported by Kyoto Research Laboratory, Matsushita Communication Industrial Co., Ltd. Yoshihiro Toshyo (1991), pp. 13-25.
As shown in FIG. 15(a), when the phase shift mask 55 utilized in this phase shift method is typically formed in a Levenson type, for example, such that the pattern of the light shielding film 552 is formed on the surface of a glass substrate 551 formed of a quartz glass or the like and a phase shift film 554 formed of a transparent thin film is formed on every other one of the transmission portions 553 where the light shielding film 552 is not provided.
When such phase shift mask 55 is utilized, as shown in FIG. 15(b), laser beams passed through the respective transmission portions 553 where the phase shift film 554 is formed are inverted in phase relative to the laser beams passed through the respective transmission portions 553 where no phase shift film 554 is formed. A mutual action acts on the boundaries between the respective passed laser beams so that they are weakened by each other at the boundaries in intensity of light. Consequently, as shown in FIG. 15(c), the light intensities at the boundaries between every adjacent two portions of the patterns on the wafer 54 become zero and the respective adjacent pattern portions are separated.
As described above, according to the phase shift method utilizing the phase shift mask 55, the phase of the passed laser beam is inverted, whereby a very fine pattern which can not be resolved by using the above-mentioned usual photo mask 52 can be resolved.
However, when the phase shift mask of a two-layer structure having the light shielding film 552 and the phase shift film 554 is manufactured, extremely high accuracy is required in a film thickness control. Hence, as compared with the photo mask 52 of single layer structure having only the light shielding film 552, the phase shift mask 55 is difficult to manufacture and involves a high manufacturing cost.
Further, the phase sight mask 55 involves a problem such that if the phase shift film 554 has a defect, even though it is a very small defect, it is transferred, as a pattern, to the wafer and therefore, it is important to inspect and correct any defect, but the inspection and correction are not easy.
Accordingly, the phase shift method utilizing the phase shift mask 55 raises a manufacturing cost and involves difficult problems to be solved, which prevents the phase shift method from being practically applied to a process of manufacturing the semiconductor devices.